So two strong boilers is too much, the steam engine just packs in after a little while but it seems to run fine on one. So there is your answer @Stalker if you’re looking for a setup without steel.
One dense bronze burning box
One strong bronze boiler
One strong bronze steam engine
One Bronze Compressor (or crusher)
and a large bronze pipe (add a brass valve for safety 
)
@Gregorius what did I calculate wrong? I was sure two was what I needed. Is there a thermometer tool analog for KU measurement?